We seek to better define the mechanism by which the A1 subunit of cholera toxin alters adenylate cyclase activity in plasma membranes and by which cyclic AMP stimulates active secretion in the small intestine. Turkey erythrocyte will be used as the principal source for adenylate cyclase. We will examine the cytoplasmic factors involved in activation, the specific binding of A1 subunit to cytoplasmic or membrane proteins and the nature of the cyclase alteration produced by toxin. V. cholera and E. coli mutants generated in other laboratories will be assayed in the erythrocyte system in order to probe the molecular specificity of key reactions in cyclase activation. Organisms producing mutant toxins will also be considered as possible candidates for a live oral vaccine against cholera. The intestinal secretory process will be probed with a variety of organic anions that compete with chloride for binding sites or transport carriers on the luminal and contraluminal epithelial cell membranes. If photoaffinity labels or other forms of irreversible binding of critical ligands can be achieved, these can also be used to characterize transport proteins.